This invention relates to an improved mechanical extensometer employing a displacement transducer for strain measurement of foil-gage metals in elevated (above 500.degree. F.) temperature tensile tests. Although optical techniques and electrical resistance strain gages are also currently accepted methods of strain measurement, each has disadvantages which make them unsuitable for elevated temperature tests on foil-gage metals. Optical techniques require direct readings by a trained operator. Such direct readings are generally subject to operator error, highly labor intensive and not amenable to automated data collection systems. Electrical resistance strain gages are commonly restricted to use at temperatures below 500.degree. F. Strain gages for use above 500.degree. F. are available, but are costly and difficult to attach properly. Also, it has been found that attachment of strain gages to foil-gage metals affects the mechanical behavior of the material. Mechanical extensometers offer the advantages of being capable of operation at temperatures in excess of 500.degree. F., ready availability, relatively inexpensive and easily incorporated into test setups using automated data collection systems. Application of mechanical extensometers to foil-gage metals, however, poses several concerns. In certain tensile tests, the additional weight of the extensometer on the specimen may greatly exceed the allowable one percent of the maximum test load. Also, the grip pressure required for attachment of the extensometer to the specimens may induce bending stresses in foil-gage materials.
There is therefore a definite need in the art for an improved mechanical extensometer for testing foil-gage metals.
Accordingly, it is an object of the present invention to provide a new and improved mechanical extensometer for strain measurement of foil-gage materials in elevated temperature tensile tests.
Another object of the present invention is an apparatus for reducing the grip pressure requirement for foil-gage materials in a mechanical extensometer.
Another object of the present invention is an apparatus for alleviating the weight contribution of a mechanical extensometer during tensile testing of elevated temperature foil-gage material specimens.
A further object of the present invention is an improved grip insert mechanism for mechanical extensometers that reduce error in tensile testing of foil-gage materials.
An additional object of the present invention is a novel counterweight arrangement for reducing the weight stress on foil-gage test specimens during tensile testing thereof.
The foregoing and additional objects of the present invention are attained by providing a pair of vertically extending connecting rods attached to each of the top and bottom furnace platens of an extensometer and slidably extending these rods through the top of the furnace wall. Individual ceramic insulators are secured to each of the rod ends that extend through the furnace wall. Tapped holes are provided approximately 180.degree. apart through the conventional top furnace platen of the extensometer for threaded receipt of one pair of the rods. The bottom furnace platen is provided with integrally secured extensions and is also provided with a pair of 180.degree. spaced tapped openings therethrough for threadingly receiving the other pair of connecting rods. The connecting rods are thus disposed such that the rods are essentially positioned at right angles to each other. The four ceramic insulators are connected by flexible wire, through a pulley system, to four weights. Each pair of weights is designed to offset the weight contribution of one-half the weight applied by the mechanical extensometer. The pulley system is suspended from a main support block integrally attached to a back support plate that, in turn, is fixed to the tensile test frame. The main support block is provided with vertically adjustable mechanisms to permit correct positioning of the connecting rods extending through the furnace top.
Novel grip inserts are provided on the top and bottom furnace platens to further reduce the grip pressure required for attachment of the specimen and to reduce bending stresses induced in the foil-gage test specimen as a result of extensometer attachment. These inserts are secured to the adjustable faces of each platen through which the test specimen is positioned. One insert in each platen is provided with a flat edge surface and the facing insert is a precision conical tip.